The present invention relates to an image processor which reproduces a composite document consisting of both binary images such as characters and half-tone images such as graphic images by identifying a character area and a graphic area during pre-scanning and switching parameters every area during main scanning.
FIG. 12 shows the configuration of a digital color image processor; FIG. 13 shows an exemplary configuration of a conventional edge processing circuit; and FIGS. 14(a) to 14(c) show the configuration of a hue detection circuit.
In a digital color copying machine, a document is usually read optically with a line sensor to obtain image data in the form of color separation signals of B (blue), G (green) and R (red), and, as shown in FIG. 12, the obtained image data are converted to toner color signals Y (yellow), M (magenta) and C (cyan) through an END (equivalent neutral density) conversion circuit 31 and a color masking (color correction) circuit 32. Further, a toner signal X of a development color is selected after replacing, for black print generation, the Y, M, C signals of the same quantity with a signal K (black) by an UCR (under color removal) circuit 33. Then, the selected toner signal X is subjected to a smoothing process and an edge enhancement process at a hue separation type nonlinear filter section, and to a tone adjustment process at a TRC (tone reproduction control) circuit 40. The signal X thus processed is converted to binary data at a screen generator (SG) 41, and this binary data is used to control a laser beam so that a charged photoreceptor is exposed on and off. A full-color document is reproduced by superposing dot images of the respective colors.
In the color image reproduction, such a digital color image processor necessarily requires a large-capacity memory if it is constructed so as to store full-color data for 4 development processes obtained by one document-reading scanning. To avoid this problem, the document-reading operation by the main scanning is performed every development color, and the resultant image data is subjected to the signal processing and then to the development process. Prior to the main scanning, pre-scanning is performed to detect the size of a document and judge whether the document is a color document or a monochrome one. Based on the information obtained by the prescanning, such operations as the copy operation control and parameter switching are performed so that a full-color output process is applied to color documents and a black (K) output process is applied to monochrome documents. To binary image documents consisting of characters and line drawings, edge enhancement is applied to improve the sharpness of images, while to half-tone documents such as photographs and dot prints, a nonlinear filtering process is applied to smooth images to improve the smoothness and granularity.
Then, the phase separation type nonlinear filter section for improving the reproduction performance of binary images and half-tone images will be described. The UCR circuit 33 selects, in accordance with the development process, development color image data X from among Y, M, C and K signals which have already been subjected to the black print generation and under color removal. The hue separation type nonlinear filter section receives the image data X and bifurcates it. A smoothing filter 34 performs a smoothing process on one image signal, while a gamma conversion circuit 36, an edge detection filter 37 and an edge enhancement LUT (look-up table) 38 perform an edge enhancement process on the other image signal. As a final stage, an adder 39 synthesizes the two outputs to produce a nonlinear filter signal. FIG. 13 shows an exemplary configuration of such an edge processing circuit.
In the edge processing, a hue detection circuit 35 detects the hue of an input image, and judges whether or not the current development color is a required color for the image. If the input image is black, the control is performed so that the Y, M and C color signals are subjected to no enhancement but only the K signal is enhanced commensurate with an edge enhancement degree.
As shown in FIG. 14(a), the hue detection circuit 35 includes: a maximum/minimum circuit 42 for calculating a maximum and a minimum of the Y, M and C signals; a multiplexer 43 for selecting a development color; a subtracting circuit 44 for calculating the difference between the maximum and the minimum; a subtracting circuit 45 for calculating the difference between the minimum and the development color; and comparators 46 to 48. Each of the comparators 46 to 48 compares its input value with a threshold and, if the input value is greater than the threshold, sets its output, such as r, m, c', m' and y', to a logic value "1". These outputs are used to obtain a Judgment hue based on the judgment conditions shown in FIG. 14 (b), and it is further judged whether the development color is a required color "1" or a non-required color "0" based on the required color/non-required color judgment conditions shown in FIG. 14(c). Ordinarily used character colors, Y, M, C, B, G, R and K are employed as the judgment hues.
As is apparent from the required color/non-required color judgment conditions, if the hue is, e.g., B, the development colors m and c are selected as the required colors while the other development colors as non-required colors. The development color image signal is subjected to the edge enhancement by using an edge enhancement LUT 38-(1) during the required color cycle while it is not subjected to the edge enhancement by using an edge enhancement LUT 38-(2) during the non-required color cycle.
However, in the above edge processing circuit, the edge enhancement LUT is controlled by specifying binary image areas for the edge enhancement in advance and generating an area signal in accordance with such specification. When processing binary images such as characters and line drawings and half-tone images such as photographs and dot prints, optimal parameters can be selected by specifying the type of image on a document or an area basis, as long as it is easy to specify a document or area in advance, and this hence contributes to improving the reproduction performance of the image. However, this step of area specification complicates the reproduction process if it is not easily conducted. Thus, if the area specification is cumbersome for a composite document consisting of both binary images and half-tone images, parameters which allow both types of images to be reproduced with moderate performance may be selected. In other words, however, neither types of images are processed optimally, thereby making it difficult to have both types of images reproduced satisfactorily. For example, a binary image becomes blur or unclear due to weak edge enhancement, and turbidity occurs in small black characters and at the edge of black characters. On the other hand, because of the enhancement of frequency components near the edge detection frequency, a half-tone image loses smoothness, becomes a coarse image having undesired moires and edge enhancement.
Thus, to identify the image type, there have been proposed a method in which a means for extracting a black component is provided and character area judgment is performed on the extracted black data (e.g., Japanese Patent Application Unexamined Publication No. Hei. 1-95673); a method using an average, standard deviation, etc. of pixels within a predetermined pixel block (e.g., Japanese Patent Application Unexamined Publication No. Sho. 63-205783); a method using binary outputs obtained by a plurality of dither conversions with different phases (e.g., Japanese Patent Application Unexamined Publication No. Sho. 63-193770); and other methods. However, these methods are neither capable of distinguishing color characters from black characters, nor capable of distinguishing characters from half-tone images, thus not contributing to improving judgment accuracy for a wide area.
Further, the above edge processing circuit has another problem that a smoothed signal remains in the signals Y, M and C even when processing a black character. This problem occurs in the following manner. As shown in FIG. 13, the edge enhancement LUT 38 just works such that the required color signal is enhanced by using the table (1) and the non-required color signal is not enhanced by using the table (2). As a result, in the case of a filter input signal of a black character, the edge enhanced signals are generated such that the signal K is enhanced, but not the signals Y, M and C, while the smoothed signals are generated in the smoothing filter 34 such that all the signals Y, M, C and K are smoothed. When the edge enhanced signal and smoothed signal are synthesized, the smoothed signals Y, M, C, K remain. Thus, despite the black character, not only the signal K but also the signals Y, M and C are carried, and this causes a color derived from the smoothed signals Y, M, C to appear at the edge portions of the character, disturbing reproduction by black (K). Comparing this example with a reproduction using only one color of black (K), there occur thickened lines, and edge discoloration and turbidity due to staggered registration, which results in impaired image quality with reduced sharpness.